The present invention relates to a method of manufacturing a buried heterostructure (abbreviated as "BH") semiconductor laser device which uses layer disordering due to impurity diffusion.
Conventionally, there are known various semiconductor laser devices and one of them is a BH semiconductor laser device using layer disordering due to impurity diffusion.
Description will be given below of an example of a conventional method of manufacturing a BH semiconductor laser using layer disordering due to impurity diffusion with reference to FIGS. 8 to 11. This manufacturing method is proposed by the present applicant and is applied for patent as Japanese Patent Application No. Hei. 4-206,465.
In the conventional method, at first, on an n-type GaAs substrate 1, a clad layer 2 formed of Se doped Al.sub.0.6 Ga.sub.0.4 As and having a thickness of 1 .mu.m, an optical waveguide layer 3 formed of undoped Al.sub.0.3 Ga.sub.0.7 As and having a thickness of 0.1 .mu.m, a quantum well active layer 4 formed of undoped GaAs and having a thickness of 0.01 .mu.m, an optical waveguide layer 3 of undoped Al.sub.0.3 Ga.sub.0.7 As and having a thickness of 0.1 .mu.m (In the drawings, the optical waveguide layer 3 and the active layer 4 are shown as the same layer.), a clad layer 5 formed of Mg doped Al.sub.0.6 Ga.sub.0.4 As and having a thickness of 1 .mu.m, and a contact layer 6 formed of Mg doped GaAs and having a thickness of 0.1 .mu.m are put one on top of one another sequentially according to the MOCVD method. Then, as shown in FIG. 8(a), an Si film 7 having a thickness of 10 nm is deposited as an impurity diffusion source film on top of the contact layer 6 by use of an electron-beam evaporation, and after then, as shown in FIG. 8(b), an SiO.sub.2 film 8 having a thickness of 50 nm as an insulation film and an Si film 9 having a thickness of 10 nm as an etching preventive film are sequentially deposited on top of the Si film 7 by use of an electron-beam evaporation. The Si film 9 is provided as an etching preventive film for the SiO.sub.2 film 8 in order to etch and remove a diffusion protect film 10 (which will be described later) formed of Si0.sub.2 selectively from the SiO.sub.2 film 8.
Then, as shown in FIG. 8(c), a resist 13 is put on top of the Si film 9 and a stripe-shaped window having a width of 5 .mu.m is formed in the resist 13 in a machining step using photolithography. And the resist 13 is used as a mask to remove the Si film 7 serving as the impurity diffusion source film, the SiO.sub.2 film 8 serving as the insulation film, and the Si film 9 serving as the etching preventive film by means of dry etching, thereby forming a stripe-shaped window as shown in FIG. 9(a). After then, as shown in FIG. 9(b), the resist 13 is removed by use of acetone and the portion from which the resist 13 has been removed is rinsed by use of isopropyl alcohol and is then washed by pure water. Then, as shown in FIG. 9(c), the whole surface of the thus formed window is covered with an SiO.sub.2 film 10 which is a diffusion protect film of 50 nm in thickness. The thus formed layer assembly is then sealed into a quarts tube together with arsenic and, after then, the assembly is thermally treated for 2 hours at a temperature of 850.degree. C. in an electric furnace to diffuse Si in such a manner as shown in FIG. 10(a), thereby forming an Si diffusion area 14. Next, as shown in FIG. 10(b), after thermally treated, the SiO.sub.2 film 10 is etched by means of buffered fluoric acid. In this case, since the Si film has an etching rate which is one-tenth or less than that of the SiO.sub.2 film, the SiO.sub.2 film 10 serving as the diffusion protect film can be removed almost selectively. After then, the assembly is sealed into the quarts tube together with zinc and arsenic and is thermally treated for 20 minutes at a temperature of 550.degree. C. and, as shown in FIG. 10(c), the Si film 7, SiO.sub.2 film 8 and Si film 9 are used as masks to diffuse Zn, thereby forming a Zn diffusion area 15. Next, as shown in FIG. 11, the Si film 7, SiO.sub.2 film 8 and Si film 9 are used as current preventive layers to thereby vapor deposit a p-side electrode 11. Also, an n-side electrode 12 is vapor deposited on the n-type GaAs substrate 1 side.
According to a semiconductor laser having the structure shown in FIG. 11, contact between the GaAs contact layer 6 serving as the non-diffusion area of the impurity and the p-side electrode 11 can be achieved with high accuracy to thereby improve contact between the electrode and the semiconductor layer. Also, between the clad layer 5 and Si diffusion area 14 there is formed a pn junction which has a turn-on voltage greater than the active layer 4 in the active area, whereby the diffusion of a current in the lateral direction can be advantageously restricted.
In the above-mentioned conventional semiconductor laser manufacturing method, in order to selectively remove the SiO.sub.2 film 10 serving as the diffusion protect film, the Si film 9 is disposed, as the etching preventive layer, on the SiO.sub.2 film 8 serving as the insulation film. However, when the diffusion window is formed, if the resist 13 is applied directly onto the Si film 9 and the working step according to the photography is executed (see FIG. 8(c)), then the residual resist is easy to occur when the resist is peeled off and the residual resist attaches to the semiconductor laser device as an impurity because it is heated when the Si is diffused, thereby lowering the yield thereof. Although the reason of the occurrence of the residual resist is not known so far, such phenomenon has been confirmed from experiments. It seems that this is because there exists a strong chemical affinity between the Si film 9 and resist 13. Also, from the viewpoint of the manufacturing process as well, the Si film must be deposited twice (see FIGS. 8(a), (b)), which is troublesome. Also, the SiO.sub.2 8 is provided in order to prevent partially the inflow of a current from the electrode 11 when the semiconductor laser is driven, but the SiO.sub.2 film raises a problem that it lowers the diffusion speed of the Si film 7 when the Si diffusion area 14 is formed.